The present invention relates to silicon nitride ceramic material, e.g., Si3N4. More specifically, the present invention relates to coatings for silicon nitride parts, which resist recession/oxidation (recession resistant coatings) in high temperature, high humidity and high gas flow environments, and methods of making the same.
Ceramic materials, which are joined by ionic or covalent bonds, are typically composed of complex compounds containing both metallic and non-metallic elements. Ceramics typically are hard, brittle, high melting point materials with low electrical and thermal conductivity, good chemical and thermal stability, and high compressive strengths. Ceramics, such as silicon nitride, offer excellent high temperature strength and creep resistance and hence are employed in a variety of high temperature demanding applications, such as bearings and valves for internal combustion engines.
Additionally, silicon nitride and other various ceramics may be used in 300 to 800 kilowatt gas turbines, e.g., micro-turbines, where the internal components are subjected to the combination of high temperature, high humidity and high gas flows. These micro-turbines are frequently used as specialized power generators for industrial applications. The internal temperatures of ceramic micro-turbines can run at higher temperatures than metal micro-turbines, e.g., 1370 degrees centigrade (C) or higher, and therefore can also run at higher efficiencies, e.g., about 15% higher.
One example of a ceramic used in a micro-turbine environment is a silicon nitride compound available from Saint-Gobain Ceramics and Plastics, Inc., of Worcester, Mass., having the trade name NT154(copyright). This material is described in U.S. Pat. No. 4,904,624, which is herein incorporated by reference in its entirety. NT154(copyright) typically is composed of 96% silicon nitride and 4% yttrium, and is also typically subjected to a sintering (densification) process to increase its density and reduce its porosity.
In a high temperature and humidity environment, silicon nitrides, and other silicon-based ceramics, will oxidize to form a thin layer of silicon dioxide (SiO2) on its surface. Over time, this silicon dioxide layer can crack or flake off to expose more ceramic to oxidation. As a result, the surface of the ceramic will slowly recede or wear away. This can be especially problematic in cases where ceramic parts must have a wear life of many thousands of hours. One such example of this is in micro-turbines which are designed to run 30,000, 45,000 or more hours.
Moreover, in micro-turbine environments, the high gas flow rates exacerbates the problem of ceramic surface recession. That is, the high gas flow continuously strips the silicon dioxide layer away to re-expose the surface to oxidation more rapidly. As a result it is possible in a micro-turbine for the surface of the ceramic to recede at rates of up to 1 mg/sq. cm hour. In other words, a square centimeter of ceramic surface, which is exposed to the environmental conditions within a micro-turbine, can potentially recede or be worn away at the rate of one milligram of thickness per hour.
A variety of environmental barrier coatings (EBC) have been employed, using such techniques as physical vapor deposition (PVD) or chemical vapor deposition (CVD), to provide a recession resistant coating for the ceramic. These barrier coatings can potentially reduce the rate of ceramic recession by upwards of an order of magnitude, i.e., a factor of 10. Unfortunately, the coatings are prone to cracking and pealing due to differences in the coefficient of expansion between the layer and the ceramic surface.
Based on the foregoing, it is the general object of the present invention to provide an environmental barrier coating for a ceramic, and method of making the coating, which overcomes the problems and drawbacks associated with the prior art.
The present invention offers advantages and alternatives over the prior art by providing a recession resistant coated ceramic part. The ceramic part has a ceramic substrate and a recession resistant coating disposed on the substrate. The coating includes a plurality of layers diffusion bonded to each other and to the substrate respectively. The top most layer is characterized by a greater resistance to recession due to oxidation than that of the substrate.
Since the layers are diffusion bonded, the problem of separation of layers is greatly alleviated. Additionally, the added resistance of the top most layer to recession due to oxidation greatly reduces the rate of recession of the over all ceramic part.
In an alternative exemplary embodiment of the recession resistant coated ceramic part, the layers graduate the effects of thermal expansion between the substrate and the top most layer. Additionally, in another embodiment, the substrate of the recession resistant ceramic part is composed substantially of silicon nitride.
An exemplary embodiment of a method of making a recession resistant coated ceramic part in accordance with the present invention includes forming a green porous ceramic part and coating the part with a plurality of layers. The top most layer is characterized by a greater resistance to recession due to oxidation than that of the part. The coated part is then surrounded in a hermetically sealable substance. The sealable substance is then made to conform to the part to form a hermetic seal thereon. The sealed and coated part is subsequently HIP (Hot Isostatically Pressed) processed to simultaneously densify the green part and diffusion bond the plurality of layers to each other and to the part respectively.
In an alternative embodiment, the hermetically sealable substance is glass and heat is applied to soften the glass such that the glass conforms to the shape of the part to form a hermetic seal over the part.